Gas turbine engines, such as those used to power modern commercial aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. In aircraft engine applications, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor. An exemplary combustor features an annular combustion chamber defined between a radially inward liner and radially outward shell extending aft from a forward bulkhead. The radially inward liner forms a heat shield. The radially outward shell extends circumferentially about and is radially spaced from the inward liner. Arrays of circumferentially distributed combustion air holes penetrate the outward shell and the inward liner at multiple axial locations to admit combustion air into the combustion chamber along the length of the combustion chamber. A plurality of circumferentially distributed fuel injectors and associated swirlers or air passages are mounted in the forward bulkhead. The fuel injectors project into the forward end of the combustion chamber to supply the fuel. The swirlers impart a swirl to inlet air entering the forward end of the combustion chamber at the bulkhead to provide rapid mixing of the fuel and inlet air. Commonly assigned U.S. Pat. Nos. 7,093,441; 6,606,861 and 6,810,673, the entire disclosures of which are hereby incorporated herein by reference as if set forth herein, disclose exemplary prior art annular combustors for gas turbine engines.
Combustion of the hydrocarbon fuel in air inevitably produces oxides of nitrogen (NOx). NOx emissions are the subject of increasingly stringent controls by regulatory authorities. One combustion strategy for minimizing NOx emissions from gas turbine engines is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL combustion strategy recognizes that the conditions for NOx formation are most favorable at elevated combustion flame temperatures, i.e. when the fuel-air ratio is at or near stoichiometric. A combustor configured for RQL combustion includes three serially arranged combustion zones: a fuel-rich combustion zone at the forward end of the combustor, a quench or dilution zone that involves the conversion of rich combustion to lean combustion, and a lean combustion zone axially aft of the quench or dilution zone. Thus, the combustion process in a combustor configured for RQL combustion has two governing states of combustion: a first state in the forward portion of the combustor that is stoichiometrically fuel-rich and a second state in a downstream portion of the combustor that is stoichiometrically fuel-lean.
During engine operation with RQL combustion, a portion of the pressurized air discharged from the compressor is directed through a diffuser to enter the combustion chamber through the inlet air swirlers to support rich-burn combustion. Concurrently, the fuel injectors introduce a stoichiometrically excessive quantity of fuel into the front portion of the combustor. The resulting stoichiometrically rich fuel-air mixture is ignited and burned to partially release the energy content of the fuel. The fuel rich character of the mixture inhibits NOx formation in the rich burn zone by suppressing the combustion flame temperature. It also resists blowout of the combustion flame during certain operating conditions or any abrupt transients to engine power and promotes good ignition of the combustor.
The fuel rich combustion products generated in the first zone of combustion propagate downstream where the combustion process continues. Pressurized air from the compressor enters the combustion chamber radially through a row of circumferentially spaced dilution air admission holes. The additional air admitted through these dilution air holes mixes with the combustion products from the first zone to support further combustion and release additional energy from the fuel. The air also progressively deriches the fuel rich combustion gases as these gases flow axially through and mix with the air introduced in the quench region. Initially, with the dilution air addition, the fuel-air ratio of the combustion products becomes less fuel rich approaching a stoichiometric composition, causing an attendant rise in the combustion flame temperature. Since the quantity of NOx produced in a given time interval increases exponentially with flame temperature, significant quantities of NOx can be produced during the initial quench process where the combustion is rich. As quenching continues, the fuel-air ratio of the combustion products rapidly convert through the stoichiometric state to become fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate appreciable quantities of NOx.
For example, the aforementioned commonly assigned U.S. Pat. No. 6,810,673, discloses an embodiment of a annular gas turbine combustor having a single row of corresponding positioned major dilution air admission holes in the inner shell and inner heat shield forming the inner liner and a single row of corresponding positioned major dilution air admission holes in the outer shell and outer heat shield forming the outer liner. These major dilution air admission holes are positioned axially whereby the dilution air is admitted therethrough into the combustor at the forward end of the quench zone. The major holes in the outer shell and outer heat shield are disposed in axial alignment with the major holes in the inner shell and the inner heat shield, but at circumferentially offset intervals therewith. In another embodiment, the single row of major holes in the outer liner further includes a set of smaller diameter minor air admission holes disposed in the single row of air admission holes circumferentially spaced intervals intermediate neighboring pairs the large diameter major air admission holes. In this embodiment, the single row of major holes in the inner liner may further include a set of smaller diameter minor air admission holes disposed in the single row of air admission holes circumferentially spaced intervals intermediate neighboring pairs the large diameter major air admission holes.
U.S. Pat. No. 5,934,067 discloses an embodiment of a gas turbine engine combustor bounded by axially extending inner and outer annular walls connected by an end wall including a plurality of fuel injectors disposed at circumferentially spaced intervals. In each of the inner and outer annular walls includes a first row of circumferentially spaced larger diameter air admission orifices in an upstream transversely extending plane and a second row of circumferentially spaced smaller diameter air admission orifices in a downstream transversely extending plane. The number of larger diameter holes in each of the upstream rows of larger diameter orifices in the inner and outer walls are equal in number to the number of fuel injectors and are arranged so that the large diameter orifices in the outer wall are displaced to one side of the centerline of the fuel injectors and the large diameter orifices in the inner wall are displaced to the opposite side of the centerline of the fuel injectors such that the air passing through the large diameter orifices flows counter to the swirling direction of the fuel injectors. The smaller diameter orifices are also equal in number to the number of fuel injection nozzles. In an embodiment, the respective rows of smaller diameter orifices are offset axially from the respective rows of the larger diameter orifices by an amount between one-half the diameter of the larger diameter orifice and one-half the diameter of the smaller diameter orifices.
U.S. Pat. No. 6,070,412 discloses an embodiment of a gas turbine engine combustor bounded by axially extending inner and outer annular walls connected by an end wall including two circumferential rows of fuel injectors, each row of fuel injectors including a set of N fuel injectors. Each of the inner and outer walls includes a row of circumferentially uniformly distributed primary air admission holes disposed in an upstream transverse plane extending perpendicular to the axis of symmetry. Each of the inner and outer walls includes row of circumferentially uniformly distributed dilution air admission holes disposed in a downstream transverse plane perpendicular to the axis of symmetry. The number of primary air holes in the upstream row in each of the inner and outer walls is equal to twice the number of fuel injectors in each row of fuel injectors. The number of dilution air holes in the downstream row in each of the inner and outer walls is equal to twice the number of primary air holes in the upstream row of the holes. The distance separating the upstream row of air holes and the downstream row of air holes in the outer wall is shorter than the distance between two consecutive primary air admission holes in the outer wall. The distance separating the upstream row of air holes and the downstream row of air holes in the inner wall is shorter than the distance between two consecutive holes in the upstream row of holes in the inner wall.
Finally, the deriched combustion products after quench flow axially into the downstream of the combustor where the combustion process concludes as lean-burn combustion. Additional jets of compressor discharge air may be admitted radially into the lean burn zone. The additional air supports ongoing combustion to complete combustion of the fuel and to reduce the peak temperature, as well as regulate the spatial temperature profile of the combustion products prior to entering the turbine. Regulation of the peak temperature and temperature profile protects the turbine from exposure to excessive temperatures and excessive temperature gradients.
High-temperature zones of localized, near-stoichiometric combustion conditions, commonly called hot spots, can occur despite the fuel-rich nature of the forward portion and the fuel-lean nature of the aft portion of a RQL combustion chamber. It is desirable to quickly quench hot spots, not only to reduce NOx production, but also to reduce temperature variation in the combustor exit gases so as to provide a relatively uniform temperature profile in the combustion gases exiting the combustor to enter the turbine of the engine.